Background Field
Embodiments of the subject matter described herein are related generally to determining a position for a mobile device, and more specifically for determining a position of a mobile device based on detected chemicals in the environment.
Relevant Background
Obtaining accurate position information for mobile devices, such as cellular telephones or other wireless communication devices, is becoming prevalent in the communications industry. A common means to determine the location of a device is to use a satellite position system (SPS), such as the well-known Global Positioning Satellite (GPS) system or Global Navigation Satellite System (GNSS), which employ a number of satellites that are in orbit around the Earth. One limitation of current SPS systems is that their operation is limited to situations in which multiple satellites are clearly in view, without obstructions. For example, obtaining a traditional SPS position fix while inside a building is typically difficult, if not impossible. Thus, once inside a building, a terrestrial positioning technique will be used. Terrestrial positioning techniques rely on data from non-SPS sources, such as wireless signals obtained from a wireless local area network (WLAN) or WiFi, or vision based techniques.
Obtaining a position fix using a traditional SPS system or terrestrial navigation techniques, however, may be time consuming and may drain battery power. For example, when a mobile device moves between indoors and outdoors, the mobile device may try to use an inappropriate or unnecessary positioning technique (e.g., SPS while indoors) for a period of time before changing to a different appropriate positioning technique. Thus, there may be a delay in determining an accurate position using the most appropriate technique. Hysteresis in the time for the mobile device to accurately determine when to switch between the indoor navigation mode and outdoor navigation mode is present as the threshold for determining which mode to use is not always clear. For example, a WLAN can broadcast several hundreds of meters. Accordingly, a mobile device may determine incorrectly that it is inside a building based on a received WLAN signal, when the mobile device is outside but relatively near the building. This uncertainty period can cause a delay in obtaining an accurate position fix using the appropriate location technique. Moreover, the mobile device will run both the indoor and outdoor systems in order to determine when to switch between the two positioning modes. Running systems for both modes, however, quickly drains battery life. Thus, reducing the transition time between modes is desirable.
Additionally, as is well known, an SPS positioning system requires that the mobile device lock on to the signals from a number, e.g., at least four, satellites. Thus, if the mobile device is moved a significant distance while the SPS positioning system is turned off, e.g., when a user travels on an airline to another city, a cold start may be required. A cold start may take considerable time, as long as twelve minutes, because the receiver must search for a satellite lock by running through all of the codes and frequency combinations until it locks on a satellite. To avoid cold starts, some mobile devices have the capability to download the current almanac and ephemeris data from a network to save acquisition time. This is called assisted GPS (A-GPS). However, some devices are still stand alone GPS devices. For standalone devices, it is desirable to have an approximate position fix so that the device can lock onto satellites and obtain an accurate position fix more quickly than if using a cold start. Additionally, assisted GPS mobile devices may not have network services in other countries. Thus, there may be situations where even an A-GPS device may be forced to perform an undesirable cold start. Thus, improvements for position determination of mobile devices are desired.